1. Field of the Invention
The present invention relates to systems and methods for synthesizing nanoparticles, and, more particularly, to such systems and methods utilizing microwave energy.
2. Description of Related Art
New applications and technology for nanoscale semiconducting and metallic nanoparticles have grown owing to advancements in the chemical synthetic methodologies for their preparation. These materials are being utilized in applications including as bio-markers, in solar cells, and in lighting technologies, for example. As nanoscale devices become more of a commercial reality, the industrialization of nanoscale materials has been limited by the need for new material compositions and the development of high-throughput automation for materials preparation.
The formation of nanocrystals is notorious for its difficulty and required long reaction times, on the order of hours. Further, for large-scale reactions inhomogeneities in the growth process can be magnified by thermal gradients in the reaction, which produce poor nucleation processes and therefore broadened size distributions. The inhomogeneity of large-scale reactions can be traced, for example, to the inefficient transfer of thermal energy from the heat source.
The growth of nanomaterials is dependent on the thermodynamic and kinetic barriers in the reaction as defined by the reaction trajectory, and is influenced by vacancies, defects, and surface reconstruction events. For the most part, the synthetic methods utilize conventional convective heating owing to the need for high-temperature-initiated nucleation followed by controlled precursor addition to the reaction. Conventional thermal techniques rely on conduction of blackbody radiation to drive the reaction, wherein the reaction vessel acts as an intermediary for energy transfer from the heating mantle to the solvent and finally to the reactant molecules. This can cause sharp thermal gradients throughout the bulk solution and inefficient, non-uniform reaction conditions. This is a common problem encountered in chemical scale-up and is made more problematic in nanomaterials where uniform nucleation and growth rates are critical to material quality.
New approaches for synthesis have been sought, particularly for controlled growth. Recent synthetic advancements have included use of non-solvents and simpler reactants, the use of single-source precursors, and microfluidic reactors. Even household microwave ovens have been used to form nanoparticles, although the resulting crystallinity and the quality of the optical properties appear to be lower in material formed by such systems.
The present inventors have previously demonstrated that microwave heating of organometallic precursors enhances growth of semiconductors, allowing the isolation of large quantities of high-quality photoluminescent semiconducting nanoparticles. The addition of additives and choice of solvent can have a dramatic effect on the observed growth behavior in the microwave by overcoming reaction barriers.
Early findings have indicated that superheating of the solvent and vessel pressurization give rise to a 1000-fold increase in reaction rate. These findings were limited to organic chemical reactions, and had not been applied in the nanocrystal field.
It has also been shown that high-quantum-efficiency CdSe samples could be prepared with quantum yields (QY) on the order of 80%; however, these routes require long reaction times and high-temperature injection.
Therefore, it would be beneficial to provide a scalable, high-efficiency, high-yield process for the production of nanoparticle materials.